1. Field of the Invention
The present invention relates to a sheet processing apparatus having a stapling function for stapling a sheet bundle.
2. Description of the Related Art
Conventionally, a post-processing apparatus has been widely known that staples a bundle of sheets having images formed thereon by an image forming apparatus. In such a post-processing apparatus, a sheet bundle cannot sometimes normally be stapled due to a failure in staple conveyance, absence of staple in a stapler, or the like. In that case, sheet bundles not normally stapled and sheet bundles normally stapled are mixedly stacked on a sheet discharge tray.
To obviate this, a conventional post-processing apparatus disclosed in e.g. Japanese Laid-open Patent Publication No. H03-121897 is configured to determine whether or not a sheet bundle has been bound with staples with their tip ends punched through the back of the sheet bundle, thereby determining whether or not the sheet bundle has been normally stapled. Sheet bundles not normally stapled are discharged to a sheet discharge position shifted on the sheet discharge tray in a direction perpendicular to a bundle conveyance direction from a sheet discharge position for normally stapled sheet bundles.
To shorten a time period required to determine whether or not a sheet bundle has been normally stapled and required to discharge the sheet bundle, it appears possible to make such determination while the sheet bundle is being discharged. In general, however, whether or not a sheet bundle has been normally stapled is determined before the start of discharge of the sheet bundle. In order to shift a sheet bundle during the discharge of the sheet bundle, an additional mechanism dedicated to a shift operation is needed, resulting in an increase of a post-processing apparatus size and an increase of apparatus manufacturing cost.